Automated force controlled assembly utilizing a novel hexapod robotic manipulator

Within industry many repetitive operations, such as pick and place, spot welding, and spray-painting, have been successfully automated. While the operations themselves are diverse, a common thread within nearly all of them is that the robot performing the task has very limited direct physical contact with its environment. From a control perspective, this means that the robot need only be controlled to follow a desired trajectory fairly precisely nteractive forces with the environment can be essentially ignored. Assembly tasks are fundamentally different in nature - while some degree of precision in positioning is needed (e.g., alignment and part pickup), successful completion is rather a function of the precision in controlling, often delicately, the forces of interaction with the environment. Common examples include gear and spline meshing, threading fasteners, "snap" fittings, etc. Humans perform these types of tasks relatively easily, as we quite naturally control our interactive forces, but typical industrial robots do not possess such compliance, and fail miserably at these tasks. Delicate, dexterous control of physical interaction forces is typically limited by manipulator mechanical architecture (serial robots have high friction and inertia), the inability to sense forces accurately, and the lack of control sophistication to achieve the desired performance when such force sensing is present. PARADEX (PARAllel DEXterous) is a novel hexapod manipulator designed specifically for these types of assembly applications. This robot offers high payload and stiffness while minimizing moving mass. It utilizes direct drive linear motors for actuation, thus limiting friction. The robot is controlled with a personal computer, a real time operating system, and a commercial off the shelf motion control board. It can be switched between precise position control and compliance control based upon whether the system is in free space or in contact with the environment. PARADEX development is part of a joint venture (Flexible Robotic Assembly for Powertrain Applications, FRAPA) sponsored by a NIST Advanced Technology Development Program. It is currently assembling candidate powertrain components (9) in a prototype work cell in Detroit. The design, development, controls, and implementation of this novel robotic parallel manipulator is presented.